Method and plant for producing flat rolled products

ABSTRACT

The invention concerns a method and a plant for producing flat rolled products, in order to obtain strips having a multiple crown transverse profile that subsequently have to be divided in a longitudinal direction into strips of a smaller width; the method provides a rolling step carried out in a rolling mill comprising roughing stands and finishing stands equipped with respective work rolls, in order to supply a strip of a determinate width.

FIELD OF THE INVENTION

The present invention concerns a method for producing flat rolledproducts, such as strip, and the corresponding production plant. Inparticular, the invention concerns a method and a plant for obtainingstrip having a final transverse profile with multiple crown and withoptimal geometric characteristics in terms of profile and planarity ofthe strip, even in the case where the strip is subsequently divided intolongitudinal portions.

The present invention can be applied in rolling processes both hot andcold, for producing strips in any type of ferrous or non-ferrousmaterials.

BACKGROUND OF THE INVENTION

Rolling plants are known, which comprise a multi-stand rolling mill,normally divided into first roughing stands and second finishing stands.A temperature restoration system may be present between the roughing andfinishing stands.

The rolling mill may or may not be disposed in line with a continuouscasting machine that produces thin slabs, the so-called “thin slabcaster”.

These plants can be designed and configured for a substantiallycontinuous rolling process, the so-called “endless” process, in whichthe cast product is rolled in a rolling mill which is located downstreamof the continuous casting machine with which it is directly engaged.

The process can also be the semi-endless type, which provides to cut thecast slab to form a plurality of coils, or the coil-to-coil type whichprovides to produce one coil at a time for each cut of the slabperformed.

It is known that the strip obtained in plants of this type normally hasa width that can vary from 600 mm to 2500 mm depending on the intendeduse of the rolled material.

However, in order to meet market requirements, there is often the needto produce coils with a narrower width than the barrel of the rollingrolls, with a consequent reduction in the productivity of the plant.

For example, if it were desired to obtain a strip width of 800 or 1000mm on a rolling plant capable of producing strip with a maximum width ofup to 2000 mm, casting a slab of width of 800 or 1000 mm, theproductivity of the plant would be substantially halved, and this is adisadvantage that should be avoided.

It is therefore known, for example from JPS 58-68405, or from JP57-175003, to work a strip of standard width, for example 1600 mm, andthen cut it in a longitudinal direction, in an inter-stand space alongthe rolling mill, to obtain two half-strips, for example 800 mm wide,which are then wound onto respective coils.

This solution, although effective in terms of maintaining productivity,has some disadvantages. The first disadvantage concerns the crown of thetwo half-strips.

The dimensional quality of the product exiting from the hot rollingprocess has as its focal point the control of the distribution ofthickness along the width of the rolled strip. The geometry of thethickness along the width of the rolled product is called profile. Themain parameter that is analyzed to evaluate the profile of the rolledproduct is the crown. The crown represents the difference between thethickness in the center and the average thickness at the edges of arolled product.

It is generally preferred to obtain a rolled product that is thicker inthe center than at the edges; therefore, seen in section, it assumes alenticular shape symmetrical with respect to the center line, as shownin FIG. 2a .

It is very important to create an accurate profile during hot rollingsince this profile cannot be modified in downstream processes, since anypossible modifications would cause defects in planarity, as well asdifficulties in carrying out the subsequent steps of the productioncycle.

On the contrary, the planarity of a rolled product is defined as itsability to adhere to a theoretical plane, and consequently non-planarityis the difference between the theoretical plane and the rolled product.

During rolling a determinate crown is imparted to the strip over theentire width by the rolling rolls, but if a strip thus conformed issubsequently divided in half, each half-strip no longer has asymmetrical crown, as can be seen in FIG. 2b : in fact the profile ofthe half-strips has a trapezoidal shape (wedge-shaped) with a differentthickness at the edges on both sides.

This asymmetrical profile, however, is not very suitable for subsequentprocessing said half-strips, making their downstream treatment unstable,with possible drift and difficulties in winding. Therefore, to have twofinished half-strips, each with its own regular profile, JP'405 providesto carry out another rolling step in another stand in order to recoverthe symmetry of the profile by tapering the cut edges.

Furthermore, in JP'405 it is problematic to carry out the longitudinalcut in an inter-stand space, and even more problematic to control thetwo half-strips, especially when dealing with thin thicknesses, due tothe high speeds involved.

The solution of JP'405 does not in practice allow to control the crownof the two half-strips since, in the single rolling stand, the profileof the half-strips is returned almost symmetrical simply by Hertzianpressure at the edges.

It can therefore be understood that in the event that the rolled stripmust subsequently be divided longitudinally into two half-strips before,or during, or even after winding, in the state of the art solutions arenot available for a precise control of the crown.

It should be noted, in this context, that the requirements of the marketfor flat products, and in particular of hot rolled strip, in recentyears have become increasingly stringent, both in terms of metallurgicalquality and also in terms of dimensional quality.

Furthermore, plant makers and steel producers are constantly seeking toreduce transformation costs while maintaining, if not improving, themechanical characteristics and subsequent workability of the hot rolledproduct.

The following aspects are correlated to the importance of thedimensional quality of the hot rolled strip:

the progressive replacement, in the production of some products, ofcold-rolled strip with hot-rolled strip;

the simplification of the production process of transforming the hotrolled strip into a finished product;

the improvement of the geometric characteristics in terms of thickness,profile and planarity. In fact, better geometric conditions make thedownstream processes more reliable and automatable, as well as improvingthe quality of the final product.

The above points lead to the requirement for “extreme” geometriccharacteristics such as for example:

crown target of the strip which, depending on the type of product, canvary from 70 μm to 10 μm. The crown, for some products (particularly forthin and ultra-thin thicknesses) must be contained within 1.0-1.2% ofthe nominal thickness of the strip. In other words, a 10 μm crown isrequired for a strip that is 1.0 mm thick;

planarity of the strip below 12 and 30 I-Units, depending on thethickness and width of the strip;

reduction of the drop in thickness at the edges (edge drop) of thestrip.

Therefore, in a production process that supplies hot-rolled strip withthin and ultra-thin thicknesses, both in endless or semi-endless andalso in coil-to-coil modes, it is necessary that the rolling stands havean adequate capacity for controlling the profile and planarity of thestrip for the entire production mix.

It is therefore known to use work rolls having a shaped form, that is,with the contour or profile that is described by a mathematical functionso that, by means of an axial shifting of the rolls in the oppositedirection, the shape of the rolling gap can be varied.

With regard to the crown, it must also be considered that the heating ofthe rolling rolls is one of the basic problems to be faced in both hotand cold rolling. The direct contact of the strip being rolled with thework rolls determines a thermal flow, with heat transfer to the rollsthemselves, and consequent heating thereof; this entails variations bothin the dimensions (diameter) and also in the profile of the rollsthemselves.

In order to limit said heating to values compatible with thecharacteristics of the material that makes up the rolls, and such as tocontain the progressive deterioration of the surface of the rolls withinacceptable values, it is essential to use cooling systems.

The solution that is generally used in hot rolling is to cool the workrolls from the outside with a series of nozzles installed on some ramps.In a conventional 4-high rolling stand for hot strip mills, four coolingdevices are generally used: two in the exit zone and two in the entryzone. Each device consists of one or more cooling ramps. To prevent theheat transmitted from the material being rolled to the roll frompenetrating from the surface layers toward the inside of the rollitself, with subsequent consequent difficulty in extracting the heataccumulated inside, it is preferred to increase the heat exchangebetween the roll and the cooling water in the exit zone from the rollinggap, increasing the flow rates and, possibly, the heat exchangeefficiency.

The heat transmitted to the rolls produces a thermal crown; an axialflow occurs since the heat, in the roll, flows from the central zone tothe two sides which, not being affected by the contact of the strip, arecolder. The result is a differentiated expansion which, generally,produces a roll profile with a quasi-parabolic shape in the centralzone, while at the edges of the strip it abruptly decreases and thenremains at lower values than those of the central zone.

The variations in the “thermal profile” of the rolls clearly affect therolling process and, in particular, the control of thickness, profileand planarity; it is therefore the task of the system for cooling therolls to minimize disturbances due to variations in the thermal profile,without prejudice to the fact that the temperatures of the rolls must onaverage reach values that vary from 50 to 80° C. (depending on thematerial that makes up the jacket of the roll) to optimize the duration,reducing surface wear caused by thermal fatigue and friction between thestrip and the roll.

These problems, already considerable in the case of hot rolling withconventional processes, are even more exasperated for endless productionprocesses, in which rolling on the strip finishing mill can last up to10 hours without interruption, compared to 2-3 minutes of theconventional coil-to-coil process.

The thermal crown of the work rolls depends on the temperaturedistribution along the axis of the roll; this distribution variescontinuously, during the rolling campaign, causing both increases anddecreases in the thermal crown with variations in the work profile ofthe roll. This phenomenon creates disturbances on the control of theprofile and planarity of the strip being rolled:

when the rolls are cold, for example after a roll change or after a longproduction pause, the thermal crown gradually increases; it takes from 5to 10 coils to reach stable values;

when stable conditions are reached during the rolling campaign, thethermal crown decreases during the waiting time between one coil and thenext, returning to the average value of thermal crown after a relativelyshort time from the start of the rolling of the new coil.

Given all of the above, one purpose of the invention is to provide amethod, and a corresponding plant, for the production of finished thinand even ultra-thin strip, which can subsequently be dividedlongitudinally in such a way as to obtain 2, 3, 4 or more distinct stripportions, each portion of the strip having optimum quality in terms ofprofile, planarity and thickness of the cross-section.

The purpose of the invention is also to maintain the productivity of therolling mill unchanged, whether a strip of width equal to the maximumwidth is being produced, or when strips of a width smaller than saidmaximum width are to be produced.

The Applicant has devised, tested and embodied the present invention toovercome the shortcomings of the state of the art and to obtain theseand other purposes and advantages.

SUMMARY OF THE INVENTION

The present invention is set forth and characterized in the independentclaims. The dependent claims describe variants or define embodiments ofthe main inventive idea.

According to one embodiment of the present invention, a slab is cast ata width defined by the design parameters of the plant itself, such asthe width of the mold, the sizing of the line, the productivityrequired, etc., and sent to a hot strip rolling mill to obtain the finalrequired thickness.

According to another embodiment of the present invention, a hot rolledstrip is further rolled in a cold rolling mill in order to obtainthinner thicknesses.

In both of these embodiments the work rolls of the stands of the rollingmill are configured to impart to the strip a transverse profile having anumber of crowns correlated to the number of longitudinal portions intowhich the strip has to be subsequently divided.

Hereafter we will use the following terminology:

“Positive” crown to indicate a symmetrical lenticular profile, thickerin the center, such as the one shown in FIG. 2 a;

“Negative” crown to indicate a bi-concave symmetrical profile, matingwith or complementary to the lenticular one, thinner at the center thanat the edges.

In accordance with the present invention, the aim is to produce two ormore positive crowns on the rolled strip by using work rolls with ashaped profile having two or more corresponding negative crowns.

The invention therefore provides to use work rolls with single crownwhen the finished strip will be used at the width of the startingproduct that is fed to the rolling mill, while work rolls with double,triple, quadruple or in any case multiple crown will be used when therolled strip has to be subsequently divided longitudinally into two,three, four or in general into a certain number of longitudinal portionsof strip.

The longitudinal division of the strip can occur along the entire lengthof the strip, from head to tail, in a position between the exit from thelast stand and respective distinct winding units on which the individualcoils of the portions of strip are formed, or it can occur along theentire length except for a segment of the head and tail of the stripimmediately before the winding onto a single coil, or it can occur afterremoving the coil from the winding unit, for example in the destinationsite of the coil itself

According to the present invention, at least the last stand of therolling mill, for example the last stand of the finishing mill, or thelast two or three stands of the finishing mill, comprise work rollswhose contact surface with the strip has a shaped profile that iscorrelated and dependent on the portions of the strip that will besubsequently obtained with the longitudinal cut.

In other words, the profile of the work rolls will have a doublenegative crown if the strip will be divided longitudinally into twohalf-strips (double crown), it will have a triple negative crown if thestrip will be divided longitudinally into three portions of strip(triple crown), and so on.

It is known from literature that the profile of the work roll can bedefined by a curve consisting of an anti-symmetric trigonometricfunction and a 3^(rd) order polynomial function.

The equations of the profile's curve are as follows:

D _(t)(y)=D−C sin α/b(y−δ _(s)−δ₀)+a ₁(y−δ _(s)−δ₀)+a ₃(y−δ _(s)−δ₀)³

D _(b)(y)=D+C sin α/b(y+δ _(s)+δ₀)+a ₁(yδ_(s)+δ₀)+a ₃(y+δ _(s)+δ₀)³

wherein:

Dt(y) is the diameter of the upper work roll;

Db(y) is the diameter of the lower work roll;

D is the nominal diameter of the work roll;

is the angle of the modifiable shape of the curve of the gap between therolls;

b is the barrel length of the work roll;

C is the amplitude of sine curve;

δ0 is the value of the primary displacement of the shaped curve of theroll;

δs is the value of the relative movement from the primary position;

a₁ is a first coefficient;

a₃ is a second coefficient.

In particular, the amplitude “C” refers to the width of the singlecrown.

The value of the crown can also be modified by varying the value δ0 ofthe axial movement (shifting) of the work rolls and, by varying theparameters α and C of the above formulas , the crown function of the gapbetween the rolls will determine a group of different curves.

Therefore, according to the present invention, by assigning suitablevalues to the coefficients a and C of the formulas above, it is possibleto obtain the “double crown” profiles, in the event the strip producedis divided into two half-strips, or even triple or quadruple crown, ingeneral multiple crown profiles, in the event the strip is dividedlongitudinally into several portions.

As mentioned, the operation of imparting to the strip a double (ortriple, or quadruple, . . . ) crown is performed in the last stands ofthe finishing mill, for example in the last or in the last two or three,in the event of particularly thin thicknesses.

It should be noted that in a finishing mill with five or six or sevenfinishing stands, the last three stands generally have work rolls withthe same diameter and the same profile. Therefore, according to theinvention it is convenient to make the multiple crown in the last threestands of the finishing mill

The invention therefore provides to produce a finished strip withmultiple crown, which is subsequently divided longitudinally in such away as to obtain multiple distinct strip portions, each with its owncrown as if they were rolled individually.

In this way, each portion of the strip has the correct crown to obtainthe desired geometric and dimensional characteristics in terms ofthickness, profile and flatness.

According to the invention, for a more accurate control of the multiplecrown, in addition to the mechanical crown of the work rolls, anintervention is also carried out on their thermal crown with a coolingmethod described below.

In the case of double crown rolling, according to the invention it isadvantageous to have minimum cooling efficiency around the central zoneof the work roll, so that the thermal crown increases in this zone,where the strip will then be divided, and instead have maximum coolingefficiency in correspondence with the central part of the two halves ofthe strip, so that the thermal crown decreases in the central zone ofthe half-strip. In other words, the thermal crown of the work roll iscontrolled so that it follows the trend of the mechanical crown,enhancing it. In the same way, also for the triple, quadruple, etc.,crown the cooling of the work rolls is modulated in a similar manner,cooling less where the strip will be divided and cooling more in thecentral zones of the respective multi-strips.

The control of the cooling system is essentially achieved through anon-line model which, at time intervals, processes a series ofinformation on the status of the process (temperatures of the strip,rolling forces, thickness reductions, rolling speed, etc.) thusdetermining the thermal profile.

The possibility of modifying the cooling efficiency on the widthaccording to the invention allows to define, along the double or ingeneral multiple crown rolling campaign, the optimal thermal crown, suchas to maximize the profile/flatness control capacity on the portions ofstrip which will then have to be divided.

BRIEF DESCRIPTION OF THE DRAWINGS

We will now describe, in detail, this and other characteristics of theinvention, with reference to some of its particular embodiments, givenas a non-restrictive example with reference to the attached drawingswherein:

FIG. 1 shows an example of a lay-out of a hot strip rolling plant onwhich the production method according to the invention is applicable;

FIGS. 2a and 2b show, respectively, a section of one strip and of twohalf-strips obtained by means of a longitudinal cut of the stripaccording to the state of the art;

FIGS. 3a and 3b show, respectively, a section of one strip and of twohalf-strips obtained by means of a longitudinal cut of the stripaccording to embodiments of the present invention;

FIGS. from 4 to 15 show graphs representative of the profiles of thework rolls, and of the resulting corresponding profiles of the strip, inthe case, respectively, of a double, triple or quadruple crown profilefor a strip width of 2000 mm and in the case of a double crown profilefor a strip width of 1600 mm;

FIG. 16 shows a graph of the trend of the angle a as a function of thewidth of the strip being worked in the case of a double crown;

FIG. 17 shows an embodiment of a differentiated cooling system of thework rolls used in the production method according to the presentinvention;

FIG. 18 schematically shows the positioning of the nozzles of thecooling system with respect to the work rolls.

DESCRIPTION OF SOME PREFERRED EMBODIMENTS OF THE INVENTION

With reference to FIG. 1, an example is shown of a co-rolling plant 10for producing strips S, in which a machine 11 for casting thin slabsfeeds a hot strip rolling mill 12.

It should be noted that the example shown is not to be considered in anyway as limiting the applicability of the present invention, since theconcepts set forth find application in a number of other types of plant,with a different number of stands, with casting separated from therolling mill, with slabs produced elsewhere and in any case in allsituations in which a metal strip, having a determinate nominal width atthe end of the rolling, has to be divided longitudinally into severalportions in order to obtain strip portions with smaller widths.

While the embodiment disclosed in the figures represents a hot rollingmill provided in line with the casting machine, the present inventioncan be applied also to cold rolling mills in which a strip obtained by aprevious step of the hot process is rolled.

In the case of hot rolling process, as mentioned, the startingsemi-finished product is represented by a slab that can be cast in-lineon the same plant (as disclosed in the embodiment of FIG. 1) or producedoff-line or in another plant.

In the case of a cold rolling process, the starting semi-finishedproduct is represented by a coil of rolled strip previously produced ina hot rolling mill According to the invention, in the cold rolling, inorder that the rolling rolls are able to impart multiple crowns to apreviously hot rolled strip having a single-crown, it is preferable thatthe thickness of the strip is at least 2.5 mm Below this value, it ispreferable that the strip to be cold-rolled does not have a single crownprofile, but has already the number of the final crowns to be obtainedat the end of the cold rolling. In this case, in the cold process, therolling rolls are shaped to follow the multicrown profile that has beenalready imparted to the strip in the previous hot rolling process.

Moreover, the present invention can be applied for the production ofboth ferrous, such as steel, and non-ferrous, such as aluminum, strips.

In this specific example shown in FIG. 1, the rolling mill 12 comprisesa roughing unit 13 (or roughing mill), comprising in this case threestands 14 a, 14 b and 14 c, and a finishing unit 15 (or finishing mill),comprising in this case five stands 16 a, 16 b, 16 c, 16 d and 16 e.

Between the roughing unit 13 and the finishing unit 15 there is atemperature restoration system, for example an induction furnace 20,which takes the slab at exit from the roughing unit 13 back to thecorrect rolling temperature.

Between the casting machine 11 and the roughing unit 13 there is atunnel furnace 17 having a length sufficient to contain at least anumber of slabs comprised between 2 and 5. This tunnel furnace 17allows, in a known manner, both to function as buffer in the event therolling mill is interrupted, even temporarily, due to accidents or aplanned change of the work rolls, and also to operate in semi-endlessmode.

Upstream of the tunnel furnace 17 there is a first pendulum shears 18,which cuts the slab to size when the plant 10 operates in coil-to-coilor semi-endless mode.

Downstream of the finishing mill 15 there is a cooling unit 22 and asecond flying shears 19, which intervenes in the case of endless orsemi-endless rolling to separate the strip gripped on one of the twodown-coilers 21, or reels.

In accordance with one aspect of the present invention, the stripobtained is subsequently divided longitudinally (slitting), so as toobtain portions of strip having a width that is a submultiple of, or inany case smaller than, the width of the cast slab.

In this way, it is possible to obtain strips with a smaller width from asingle rolled strip without limiting in any way the overall productivityof the plant, which can always work with a width of the slab and of thestrip close to the maximum one provided for the plant itself.

The division in width of the final rolled strip can take place directlyin line, at exit from the rolling mill, or in a step following theremoval of the coil, for example in a different destination plant wherethe strips are used.

In the first case, the division downstream of the finishing mill 15, forexample into two parts, can concern:

the entire length of the strip S, from head to tail, winding twodifferent half-strips S1, S2 onto respective reels 21: in this way, twodistinct coils will be obtained;

the entire length of the strip S except a portion of the head and of thetail thereof, in order to facilitate the entry of the head into thesingle reel and the winding of the last tail-end turn: in this way,there will be a single coil sectioned into two parts for almost thetotality of its length.

For this purpose, dedicated cutting devices can be provided whichlongitudinally separate the strip S into two or more strip portions S1,S2 having the same or different width. Advantageously, these devices canbe inserted into and extracted from the production line based onrequirements.

According to the number of portions in width into which the strip S willbe divided, the present invention provides to make the profile of thework rolls 24 a, 24 b of at least some of the last finishing stands 16a-16 e so as to determine the correct crown on each of the portions intowhich the strip will be divided.

By way of example, FIGS. 3a and 3b respectively show the cross-sectionof a strip S downstream of the rolling mill 12, and the sections of thetwo half-strips S1, S2 obtained by longitudinally cutting the strip Salong the center line. In the example, the strip S has a “doublepositive crown” which is substantially symmetrical with respect to aplane of symmetry passing through the center line M, while the twohalf-strips S1, S2 each have their own single positive crown.

Therefore, in the event the strip is divided in width into twohalf-strips, the profile of the work rolls 24 a, 24 b will show a doublenegative crown, one for each half-strip obtained or obtainabledownstream, the same is the case in the event of three, four or moredivided portions of strip.

The profile of each work roll 24 a, 24 b can be defined by a curveconsisting of an anti-symmetric trigonometric function and a 3^(rd)order polynomial function.

The equations of the profile's curve are as follows:

D _(t)(y)=D−C sin α/b(y−δ _(s)−δ₀)+a ₁(y−δ _(s)−δ₀)+a ₃(y−δ _(s)−δ₀)³

D _(b)(y)=D+C sin α/b(y+δ _(s)+δ₀)+a ₁(yδ_(s)+δ₀)+a ₃(y+δ _(s)+δ₀)³

wherein

Dt(y) is the diameter of the upper work roll 24 a;

Db(y) is the diameter of the lower work roll 24 b;

D is the nominal diameter of the work roll;

is the angle of the modifiable shape of the curve of the gap between therolls;

b is the barrel length of the work roll;

C is the amplitude of sine curve;

δ0 is the value of the primary displacement of the shaped curve of theroll;

δs is the value of the relative movement from the primary position;

a₁ is a first coefficient;

a₃ is a second coefficient.

According to the present invention, by assigning suitable values to thecoefficients α and C of the formulas above, it is possible to obtain the“double crown” profiles, in the event the strip produced is divided intotwo half-strips, or even triple or quadruple crown, in general multiplecrown profiles, in the event the strip is divided longitudinally intoseveral portions.

Once the profile of the work rolls 24 a, 24 b has been determined(mechanical crown), the extent of the crown on the strip can be modifiedby varying the value δs of the axial movement (shifting) of the workrolls 24 a, 24 b, as shown in FIGS. 6, 9, 12, 15.

With reference to FIGS. 4-6, an example is shown in which the strip Shas a width of 2000 mm, corresponding to the width of the cast slab, andis rolled with a double crown by work rolls 24 a, 24 b having a barrellength equal to 2450 mm, in order to be subsequently dividedlongitudinally into two half-strips of 1000 mm It should be understoodthat these drawings represent the case in which the division is into twohalves of equal width, as shown for example in FIG. 3b , however we donot exclude that the two portions of strip can have different widths.

As an example, the last finishing stand 16 e (however, it could be thelast two, three or more), is represented in FIGS. 4a and 5a ascomprising upper 24 a and lower 24 b work rolls, and upper 25 a andlower 25 b support rolls.

FIGS. 4b-4c and 5b-5c represent the profile, respectively, of the upperwork roll 24 a and of the lower work roll 24 b, in two distinctoperating conditions.

In FIGS. 4b and 4c the profile of the work rolls 24 a and 24 b isrepresented for the entire length of the barrel by lines L(T) for theupper work roll 24 a and L(B) for the lower work roll 24 b, in anon-axially shifted condition.

In FIGS. 5b and 5c the lines L(T) and L(B), indicated with a dashedline, again represent the profile of the entire length of the barrel ofthe work rolls 24 a, 24 b, while the solid lines L(Tu) and L(Bu)represent the useful part of the profile of the work rolls which workson the strip S, in an axially shifted condition, as represented by thearrows F1 and F2 in FIG. 5 a.

Finally, FIG. 5d shows the resulting profile P(S) of the strip S as thesum of the profiles L(Tu) and L(Bu). The vertical end lines indicate thelateral edges of the strip S, while the central vertical line 26indicates the central point in which the strip S will be divided.

As can be seen from the graphs, the profile of the work rolls 24 a, 24b, and obviously the resulting profile of the strip P(S), has a “doublecrown” shape, with two humps and two corresponding troughs which createthe desired crown on the resulting profile of the two half-strips intowhich, in this specific case, the strip S will be divided.

In particular, the crown on the work rolls 24 a, 24 b is “negative”,that is, it has a concave shape, while a “positive” crown, that is,having a convex shape, is obtained on the rolled strip S.

In this way, the strip S can be divided longitudinally in correspondencewith its centerline, with the possible removal of a small central bandin order to make the crown of the two half-strips “perfectly”symmetrical.

It should be considered that the extent of the crown of the single“hump” is a function of the axial shifting of the work rolls 24 a, 24 b.

This is an advantage because the adjustment of the profile is notstatic, but can be of a dynamic type and the extent of the shifting ofthe work rolls 24 a, 24 b will vary in relation to the operatingconditions of the work rolls 24 a, 24 b.

In addition, a same profile of the work rolls 24 a, 24 b can be appliedon several stands, making them operate in different shifting fields, inorder to maintain the homothety of the section of the strip S in thelast rolling stands. This is in order to not penalize the planarity ofthe strip S itself

The graph of FIG. 6 shows how the crown of the strip S can be modifiedby acting on the shifting, that is, the axial displacement of the twowork rolls 24 a, 24 b in order to vary the surface portions of therespective roll which work directly on the strip S.

Thanks to the axial shifting of the work rolls 24 a and 24 b it ispossible to accentuate or flatten the ridges and valleys of the profileof the strip S, which means increasing or decreasing the crown of the Sstrip.

The shifting of the work rolls 24 a and 24 b is symmetrical, that is,the rolls are translated in the opposite direction with respect to thecenter line M by an equal value.

In the graphs of FIGS. 5a-5d , a shifting equal to 50 mm was considered.

FIGS. 7 and 8 represent the case in which the strip S has to be dividedlongitudinally into three portions, in this case, each having a widthequal to ⅓ of the width of the strip S.

FIGS. 7a and 8a show the upper 24 a and lower 24 b work rolls withrespective profiles having a triple negative crown.

Also in this case, FIGS. 7b, 7c represent the profile of the entirebarrel length of the work rolls 24 a, 24 b in a reciprocally non-shiftedcondition, while FIGS. 8b and 8c represent the useful work portionL(Bu), L(Tu) of the work rolls 24 a and 24 b in a condition in whichthey are reciprocally shifted by 50 mm

Number 26 in FIG. 8d represents the two sections which allow to obtainthe three portions from the strip S produced.

It can be seen how the profile of the work rolls 24 a and 24 b is shapedwith a negative crown so as to obtain a profile of the strip with atriple hump which, as can be seen in FIG. 8d , determines a resultingprofile with triple positive crown, in this case, substantiallysymmetrical with respect to the center line of each of the (three)portions into which the strip S is divided, in correspondence with thesections 26.

FIG. 9 shows, in a corresponding manner, the range of control of thecrown that can be obtained by the axial shifting the work rolls 24 a, 24b, which are shaped as shown in FIGS. 7b and 7 c.

Finally, in a manner substantially equivalent to the cases describedabove, FIGS. 10 and 11 concern the case in which the strip S producedhas to be divided into four portions, in this specific case, all with asubstantially equal width.

Without repeating the concepts expressed above, we wish to point out howthe profile of the work rolls 24 a, 24 b is shaped with a quadruplenegative crown, and in FIG. 10 it is represented in a non-shiftedcondition. FIG. 11 represents the shifted condition of the two workrolls 24 a, 24 b with the useful profile L(Tu), L(Bu) respectively ofthe upper 24 a (FIG. 11b ) and lower 24 b (FIG. 11c ) work rolls shownwith a solid line. In this case, the two work rolls 24 a, 24 b areshifted by 80 mm.

The resulting profile of the strip S (FIG. 11d ) has the four humps orpositive crowns in a substantially symmetrical position, so that, afterthe longitudinal separation of the four portions by means of thesections 26, each portion has the correct pre-established crown.

As in the previous cases, using the axial shifting allows to achievecontrol of the crown, as shown for example in FIG. 12.

With reference to FIGS. 13-15, an example is shown in which a strip S isproduced, on the same rolling mill with a barrel length of the workrolls of 2450 mm, said strip S having a width of 1600 mm, correspondingto the width of the cast slab, and is rolled always with a double crownso as to be subsequently divided longitudinally into two half-strips of800 mm.

The work rolls 24 a, 24 b shown in FIGS. 13b and 13c , in the examplecase have a shaped profile with a double negative crown with rectilinearend segments (not shaped) since the strip to be rolled now has a widthsmaller than the previous example.

FIGS. 13b, 13c represent the overall profile of the work rolls 24 a, 24b in a reciprocally non-shifted condition, while FIGS. 14b and 14crepresent the shifted condition of the two work rolls 24 a, 24 b withthe useful work profile L(Tu), L(Bu) respectively of the upper 24 a(FIG. 14b ) and lower 24 b (FIG. 14c ) work rolls represented with asolid line. The work rolls 24 a, 24 b in the example case are shifted by50 mm.

The resulting profile of the strip S (FIG. 14d ) has the two humps orpositive crowns in a substantially symmetrical position, so that, afterthe longitudinal separation of the two portions by means of the sections26, each portion has the correct crown pre-established according to thequalitative requirements demanded.

As in the previous cases, using the axial shifting allows to achievecontrol of the crown, as shown for example in FIG. 15.

As mentioned, the operation of imparting to the strip a double (ortriple, or quadruple, . . . ) crown is performed in the last stands ofthe finishing mill 15, for example in the last one or in the last two orthree, in the case of particularly thin thicknesses.

FIG. 16 shows how the amplitude of the angle a can vary as a function ofthe overall width of the rolled strip S, for example for width valuescomprised between 800 and 2000 mm in the case of a strip S having adouble crown.

As mentioned above, multiple crown rolling requires strict control ofthe cooling efficiency on the width of the work roll, so that it can beselectively varied from the center to the periphery.

According to the invention, as shown by way of example only in FIG. 17,a cooling system 30 is provided comprising one or more ramps 33 fordelivering a cooling fluid with respective main feed pipes 31 anddelivery nozzles 32 distributed over the entire width of the work rolls24 a, 24 b.

The delivery nozzles 32 are disposed adjacent to each other with adeterminate pitch in a double or triple row and are connected, ingroups, to the pipes 31, independent from each other, so as to defineindependent and differentiated cooling zones on the width of the rolls.In the example shown in FIG. 17, the ramp is divided into elevenindependent cooling zones.

Each feed pipe 31 is equipped with its own proportional valve thatregulates the flow rate to the respective group of nozzles 32.

In this way, it is possible to have a separate management of the groupsof nozzles 32 and therefore to vary the cooling on the correspondingsurface zones of the work roll 24 a, 24 b.

According to the invention, also on the basis of the portions in widthto be obtained starting from a given width of strip, each delivery ramp33 can be divided into a plurality of independent zones, for examplebetween 7 and 17. It is therefore possible to define suitable variationsof cooling efficiency, along the axis of the work roll 24 a, 24 b, inparticular in order to separately control the cooling on the two halvesof the strip, or on the three, four or more portions into which thestrip S will be subsequently divided.

For example, in the case of double crown working, it is advantageous tohave a minimum cooling efficiency around the central zone of the workroll 24 a, 24 b so that the thermal crown increases in this zone, andinstead have maximum cooling efficiency in correspondence with the zoneof the roll that operates in correspondence with the central part of thetwo halves of the strip, so that the thermal crown decreases in thiszone. In this way, the thermal crown can be controlled so that itfollows the trend of the mechanical crown.

For example, with a work roll 24 a, 24 b, for producing strips with amaximum width of 2000 mm, the width of each zone can vary from about 130mm to about 220 mm.

According to some embodiments, for example described with reference toFIG. 18, the cooling system 30 can comprise four cooling ramps 33 foreach of the multiple crown finishing stands 16 a-16 e, disposed in pairsat entry and at exit to the upper 24 a and lower 24 b work rolls.

The cooling ramps 33 can advantageously be provided with drive devices34 configured to move them toward/away from the respective work roll 24a, 24 b, or rotate them with respect thereto, in order to modify theangle of incidence of the cooling liquid on the work roll 24 a, 24 b.

According to some embodiments, the strip S can be cut longitudinally inthe processes downstream of the rolling mill 12, and then be entirelywound into coils with multiple crown profile.

According to some variants, it can be provided that the strip S is woundfor an initial head segment with a multiple crown profile, andsubsequently a cutting disk located upstream of the reel 21 is driven inorder to longitudinally divide the strip S while the winding continues.In this case, the longitudinal cutting can be interrupted before thefinal tail end, which therefore remains whole as the head, with amultiple crown profile.

It is clear that modifications and/or additions of parts may be made tothe plant and to the method as described heretofore, without departingfrom the field and scope of the present invention.

1. Method for producing flat rolled products, in order to obtain stripswith a multiple positive crown transverse profile, which provides arolling step carried out in a rolling mill comprising finishing standsequipped with respective work rolls, in order to supply a strip of adeterminate width, wherein at least the work rolls of at least the lastfinishing stand are provided to have a profile with multiple negativecrown, wherein the number of crowns present in the profile of the workrolls is correlated to the number of portions into which the rolledstrip produced is subsequently divided in a longitudinal direction. 2.Method as in claim 1, wherein the work rolls are provided to have anaxial shifting movement, and wherein said axial shifting movement allowsto modify the position of the crowns of the work rolls with respect tothe position of the strip.
 3. Method as in claim 1, wherein the lastthree finishing stands have work rolls with the same diameter and sameprofile, and wherein the profile of the work rolls with multiplenegative crown is applied in said last three finishing stands.
 4. Methodas in claim 1, wherein the equations of the curve of the shaped profileof the work rolls are as follows:D _(t)(y)=D−C sin α/b(y−δ _(s)−δ₀)+a ₁(y−δ _(s)−δ₀)+a ₃(y−δ _(s)−δ₀)³D _(b)(y)=D+C sin α/b(y+δ _(s)+δ₀)+a ₁(yδ_(s)+δ₀)+a ₃(y+δ _(s)+δ₀)³wherein Dt(y) is the diameter of the upper work roll; Db(y) is thediameter of the lower work roll; D is the nominal diameter of the workroll; α is the angle of the modifiable shape of the curve of the gapbetween the work rolls; b is the barrel length of the work roll; C isthe amplitude of sine curve; δ0 is the value of the primary displacementof the shaped curve of the work roll; δs is the value of the relativemovement from the primary position; a₁ is a first coefficient; a₃ is asecond coefficient; and wherein by acting on the parameters α and C themultiple crown profile is determined in relation to the number ofportions of strip into which the strip has to be divided.
 5. Method asin claim 1, wherein in the case of a double crown profile, there isprovided a differentiated cooling with minimum cooling intensity aroundthe central zone of the work roll and maximum cooling intensity incorrespondence with the zone of the work roll that operates incorrespondence with the central part of the two halves of the rolledstrip.
 6. Plant for producing flat rolled products, in order to obtainstrips with a multiple positive crown transverse profile, comprising atleast one unit of finishing stands with work rolls, wherein in order toobtain a strip which, in a subsequent step and in a moment that followsthe end of the rolling, will be longitudinally sectioned into multipleportions, at least the last finishing stand of the finishing unitcomprises work rolls having a multiple negative crown profile, whereinthe number of crowns present in the profile of the work rolls iscorrelated to the number of portions into which the strip will bedivided.
 7. Plant as in claim 6, wherein at least the last three standsof the finishing unit have work rolls having a multiple negative crownprofile.
 8. Plant as in claim 6, wherein at least the work rolls areequipped with an axial shifting movement, wherein the magnitude anddirection of the axial shifting movement is correlated to obtaining adesired profile to be obtained on the strip (S).
 9. Plant as in one orthe other of claims from 6, configured to operate in either one and/orthe other of endless, semi-endless or coil-to-coil modes.
 10. Plant asin claim 6, comprising a differentiated system for cooling the workrolls with cooling intensity able to be adjusted as a function of themultiple negative crown shaped profile of the work rolls.
 11. Plant asin claim 10, wherein said cooling system comprises a plurality ofcooling ramps each comprising delivery nozzles disposed adjacent to eachother, with a determinate pitch, in double or triple rows and connectedin groups to respective feed pipes, independent from each other, so asto define independent and differentiated cooling zones on the width ofthe rolls, wherein each pipe is equipped with a proportional valve thatregulates the flow rate to the respective nozzles.
 12. Work roll for afinishing stand of a rolling plant for strips, comprising a multiplenegative crown profile, wherein the number of crowns present in theprofile of said work roll is correlated to the number of portions intowhich the strip produced is intended to be divided longitudinally.